1. Field of the Invention
The present invention relates to an ophthalmologic image processing apparatus used for ophthalmologic examination in an ophthalmologic clinic and the like.
2. Related Background Art
In a conventional fundus camera, image taking is performed with an aperture mask that performs light shielding on an area other than a central region inside a flare region located at a position immediately before an image pickup plane or at an equivalent position on an optical path, in order to determine an effective image taking area by eliminating a flare caused near a formed image, to determine a positional relationship between the image and an eye to be examined, or to determine a left or right eye in a case of picking up a pair of a papillary area and a macular area. In such fundus camera, because the aperture mask is located near an imaging plane of a fundus image, a boundary between the fundus image and the mask on the taken image is natural, so that a smooth image can be obtained.
On the other hand, there exists a fundus camera that produces a fundus image without using the aperture mask. However, the fundus image taken by such fundus camera has an unnecessary flare caused in the vicinity thereof, an image taking direction thereof cannot be checked, or left or right eye may be inadvertently determined. Therefore, when printing the images thereof, various problems may occur in the diagnostic process.
Therefore, as disclosed in Japanese Patent Application Laid-Open No. H9-206278, a method of electrically adding an aperture mask to a fundus image by synthesizing processing has been proposed. In this method, image processing for replacing a signal in an aperture mask area on the taken fundus image with data of a black level is performed. In the case where the fundus image is a video signal, the signal is changed into a signal of the black level at a display timing of the aperture mask area. In the case where the fundus image is inputted as image data, a black value is written into each pixel data in the aperture mask area.
However, performing the above-mentioned image processing to add the aperture mask causes the following problem. That is, generally, the fundus image becomes a circular image or an oval image whose top and bottom portions are chipped because an image taking object is spherical and it is preferable to perform image taking for its maximum area. Therefore, the vicinities of a horizontal portion and a vertical portion of the image become a gentle curve. However, in the case where an aperture mask process is performed on such a fundus image, an aliasing (step portion) is caused in the boundary between the aperture mask and the fundus. In particular, the aliasing becomes conspicuous in the gentle curve part of the horizontal and vertical portions, so that a region having developed the steps is very noticeable due to an extremely high contrast.
FIG. 8 is an explanatory view showing a fundus image synthesized with the aperture mask. This is an image obtained by synthesizing an aperture mask with a fundus image taken with the fundus camera using no aperture mask by a conventional method.
FIGS. 9A and 9B each show an enlarged image of a horizontal portion A and an enlarged image of a vertical portion B in the boundary region between the aperture mask and the fundus image as shown in FIG. 8. As is also apparent from the enlarged images, the boundary line between the aperture mask and the fundus image is a gentle curve. Therefore, the steps are very noticeable.
Further, a viewer may visually sense the step portion as a noise to hinder diagnosing operation. Alternatively, a viewer may feel uncomfortable with the image having the aperture mask, and this becomes a factor of a viewer's fatigue in diagnosing operation in which the viewer views several tens to several hundreds of images per day. Even in the case where the step portion is viewed through either a monitor or a printed paper, the similar influences are caused. In particular, in the case where the step portion is viewed through a monitor having a small number of pixels, the influences are noticeable.
In the case where the synthesized image is transmitted through a communication unit or subjected to JPEG compression for storage, the step portion becomes a discontinuous image. Consequently, there is a problem in that a contour in which the boundary portion between the fundus image and the aperture mask appears somewhat bright develops. A false color is generated in the portion which appears bright and it is a color which is not included in the original fundus image, so that attention is required for diagnosis. In order to suppress the generation of the false color, it is necessary to reduce the degree of compression, which means that it is useless to perform the compression. Further, because the step portion has a large number of harmonic components, there is a problem in that the compression efficiency is reduced.